Following the recent IT revolution, the internet communication such as electronic mail (referred simply as e-mail hereinafter) or the like for example, are becoming essential to the modern society. On the other hand, such internet communication is being questioned of its safety and reliability due to increasing number of communication interception victims. FIG. 16 shows an exemplary way of intercepting communication, and in the drawing, an interception device called sniffer is set without authorization within the network (near the entrance of the router in this example), and shows a case of illegally intercepting valuable information such as passwords or the like.
In order to prevent such communication interception, an application of enciphered communication may be given as a countermeasure proposal. However, the current enciphered communication requires cumbersome preparation and high cost, and therefore has not spread amongst the general public. Even in cases using the enciphered communication system, such method tends to be fixed and thus does not guarantee safety from communication interception due to ease of deciphering.